The present invention refers to a system for controlling and adjusting the voltage acting on a direct current motor with the purpose of positioning a variable autotransformer and obtaining at the brushes thereof the desired output voltage. This alternating current voltage is used to obtain a high voltage, by means of a high-voltage transformer, which is applied to an X-ray tube, to obtain a radiation which is displayed on a screen or a radiographic plate to carry out a clinical study of a patient.
The system for controlling and adjusting the voltage acts on a direct current motor which is fed with positive and negative voltages, depending on the direction of turn and the braking sequence thereof.
The invention controls, with a first closed loop, the output voltage of the brushes of a variable autotransformer. The required voltage is compared with the output voltage of the brushes after the output voltage is detected and rectified, and the result of this comparison constitutes the error signal of the voltage loop which, after being corrected and amplified, controls the correct position of the brushes of the variable autotransformer.
The system controls, with a second closed loop, the current of the motor which is equivalent to a control of the torque of the motor, wherefore there is no armature saturation effect, implying an automatic control of the three adjustment phases of the servosystem corresponding to acceleration time, uniform movement and braking.
The movement of the brushes of the variable autotransformer is a function of the required voltage demand which the operator fixes in the control system of the X-ray generator and the positioning thereof takes place in a vacuum without the passages of intensity, prior to the exposure of X-rays and during a time in which an automatic compensation of the network voltage is permitted.
Typically, conventional systems for controlling the voltage in X-ray systems use positioning transducers, indirectly measuring the output voltage of the variable autotransformer. These methods are affected by the mechanical tolerances and the roughness of the autotransformers, which are not linear and are difficult to compensate.
The positioning transducer itself introduces errors in the system, due to the non-linearity and the tolerances in the accuracy of the measurement. It does not automatically compensate for the shifts in the network voltage, wherefore a stabilizer should be installed at the input of the network.
The control of the motor by a continuous or transitional speed feedback which, in short, is a control of the armature voltage of the motor, increases the time constant of the system, since it depends on the electrical and mechanical constant of the motor. This consequence is very important from the point of view of a dynamic response of the servosystem, with respect to acceleration as well as to braking. See Appendix I (Calculation of the transfer function of a direct current motor fed by voltage or by current control).
On the other hand, the open loop control of the output voltage of the variable autotransformer requires a considerable number of adjustments and supplementary circuits to obtain the desired output voltage.